Photon counting is a well-known method detecting low intensity light. However, existing approaches for photon counting suffer from nonlinearities at high photon count rates. Several strategies have been adopted for improving the linear dynamic range (LDR) of light detection using photomultiplier tube (PMT) and avalanche photodiode detectors (APD), or other detector types having similar statistic properties.
For example, neutral density filters can attenuate high light levels so as to remain within the linear range of counting systems. The response time of this technique is not fast enough to provide large continuous dynamic ranges in rapid sampling applications and requires careful calibration of optical density for the filters. Another method uses a plurality of photo detectors and fiber-optic beam splitters\to sample different fractions of the beam, equivalent to performing simultaneous photon counting with several neutral density filters. Using this approach, 6 orders of magnitude of linear been response has been achieved.
Other methods include the combining of photon-counting detection for low-light levels with analog-to-digital conversion (ADC) so as to extend the LDR to the high photon flux regime. Another method includes fast ADC of the temporal time-trace followed by Fourier transformation to deconvolve the number of photons present in a time window, but this requires long analysis times and fast ADC (˜1 GHz sampling).
Each of these approaches involves performance trade-offs. Detectors optimized for photon counting with fast rise/fall times are generally not optimized for ADC and vice versa. Sensitivity mismatch in the instrument responses from single photon counting (SPC) with ADC may impact reliable quantitation and may require simultaneous data acquisition using two fundamentally different electronics approaches. The noise contribution from combinations of multiple detectors is additive. In addition, differences in sensitivity and drift may compromise the accuracy when stitching together the results from multiple detectors.
The relationship between the detected count rate and the selection of the threshold voltage of a counting discriminator(s) has been studied. Use of multiple thresholds to improve the dynamic range of photon counting systems from a single-channel detector has been demonstrated In measurements with pulsed excitation and long times between pulses relative to the detector response time (e.g., multi-photon and nonlinear optical microscopy at <100 MHz laser pulse repetition rates with detector fall times <10 ns); the voltage transients from the single-photon events can be reliably treated as temporally coincident. Detection of up to 4 simultaneous photons per laser pulse was achieved by careful adjustment of the detection voltage threshold of each discriminator to fall between the peak voltage distributions of n and n+1 simultaneous photons. This approach suffers in practice from the relatively large intrinsic variations in peak voltage distributions for a single photon in most practical photomultiplier tubes. Since the mean and variance in the peak voltage distribution increase linearly with the number of photons, the distributions for and n and n+1 photons quickly overlap as n increases, rapidly increasing the uncertainty in attempts to quantify the number of simultaneous photons with this approach.